Ladar System with Adaptive Receiver

ABSTRACT

Disclosed herein are various embodiments for a ladar system that includes an adaptive ladar receiver whereby the active pixels in a photodetector array used for reception of ladar pulse returns can be adaptively controlled based at least in part on where the ladar pulses were targeted by the ladar transmitter.

CROSS-REFERENCE AND PRIORITY CLAIM TO RELATED PATENT APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 15/430,235, filed Feb. 10, 2017, and entitled “Ladar ReceiverRange Measurement using Distinct Optical Path for Reference Light”, nowU.S. Pat. No. ______, which claims priority to U.S. provisional patentapplication 62/297,112, filed Feb. 18, 2016, and entitled “LadarReceiver”, the entire disclosures of each of which are incorporatedherein by reference.

This patent application is also related to (1) U.S. patent applicationSer. No. 15/430,179, filed Feb. 10, 2017, and entitled “Adaptive LadarReceiving Method”, now U.S. Pat. No. 10,761,196, (2) U.S. patentapplication Ser. No. 15/430,192, filed Feb. 10, 2017, and entitled“Adaptive Ladar Receiver”, now U.S. Pat. No. 10,754,015, (3) U.S. patentapplication Ser. No. 15/430,200, filed Feb. 10, 2017, and entitled“Ladar Receiver with Advanced Optics”, now U.S. Pat. No. 10,641,872, and(4) U.S. patent application Ser. No. 15/430,221, filed Feb. 10, 2017,and entitled “Ladar System with Dichroic Photodetector for Tracking theTargeting of a Scanning Ladar Transmitter”, the entire disclosures ofeach of which are incorporated herein by reference.

INTRODUCTION

It is believed that there are great needs in the art for improvedcomputer vision technology, particularly in an area such as automobilecomputer vision. However, these needs are not limited to the automobilecomputer vision market as the desire for improved computer visiontechnology is ubiquitous across a wide variety of fields, including butnot limited to autonomous platform vision (e.g., autonomous vehicles forair, land (including underground), water (including underwater), andspace, such as autonomous land-based vehicles, autonomous aerialvehicles, etc.), surveillance (e.g., border security, aerial dronemonitoring, etc.), mapping (e.g., mapping of sub-surface tunnels,mapping via aerial drones, etc.), target recognition applications,remote sensing, safety alerting (e.g., for drivers), and the like).

As used herein, the term “ladar” refers to and encompasses any of laserradar, laser detection and ranging, and light detection and ranging(“lidar”). Ladar is a technology widely used in connection with computervision. In an exemplary ladar system, a transmitter that includes alaser source transmits a laser output such as a ladar pulse into anearby environment. Then, a ladar receiver will receive a reflection ofthis laser output from an object in the nearby environment, and theladar receiver will process the received reflection to determine adistance to such an object (range information). Based on this rangeinformation, a clearer understanding of the environment's geometry canbe obtained by a host processor wishing to compute things such as pathplanning in obstacle avoidance scenarios, way point determination, etc.However, conventional ladar solutions for computer vision problemssuffer from high cost, large size, large weight, and large powerrequirements as well as large data bandwidth use. The best example ofthis being vehicle autonomy. These complicating factors have largelylimited their effective use to costly applications that require onlyshort ranges of vision, narrow fields-of-view and/or slow revisit rates.

In an effort to solve these problems, disclosed herein are a number ofembodiments for an improved ladar receiver and/or improved ladartransmitter/receiver system. For example, the inventors disclose anumber of embodiments for an adaptive ladar receiver and associatedmethod where subsets of pixels in an addressable photodetector array arecontrollably selected based on the locations of range points targeted byladar pulses. Further still, the inventors disclose example embodimentswhere such adaptive control of the photodetector array is augmented toreduce noise (including ladar interference), optimize dynamic range, andmitigate scattering effects, among other features. The inventors showhow the receiver can be augmented with various optics in combinationwith a photodetector array. Through these disclosures, improvements inrange precision can be achieved, including expected millimeter scaleaccuracy for some embodiments. These and other example embodiments areexplained in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example embodiment of a ladartransmitter/receiver system.

FIG. 1B illustrates another example embodiment of a ladartransmitter/receiver system where the ladar transmitter employs scanningmirrors and range point down selection to support pre-scan compression.

FIG. 2 illustrates an example block diagram for an example embodiment ofa ladar receiver.

FIG. 3A illustrates an example embodiment of detection optics for aladar receiver, where the imaging detection optics employ a non-imaginglight collector.

FIG. 3B illustrates another example embodiment of detection optics for aladar receiver, where the afocal detection optics employ a non-imaginglight collector.

FIG. 4 illustrates an example embodiment of imaging detection optics fora ladar receiver, where the imaging detection optics employ an imaginglight collector.

FIG. 5A illustrates an example embodiment of a direct-to-detectorembodiment for an imaging ladar receiver.

FIG. 5B illustrates another example embodiment of a direct-to-detectorembodiment for a non imaging ladar receiver.

FIG. 6A illustrates an example embodiment for readout circuitry within aladar receiver that employs a multiplexer for selecting which sensorswithin a detector array are passed to a signal processing circuit.

FIG. 6B illustrates an example embodiment of a ladar receiving methodwhich can be used in connection with the example embodiment of FIG. 6A.

FIG. 7A depicts an example embodiment for a signal processing circuitwith respect to the readout circuitry of FIG. 6A.

FIG. 7B depicts another example embodiment for a signal processingcircuit with respect to the readout circuitry of FIG. 6A.

FIG. 8 depicts an example embodiment of a control circuit for generatingthe multiplexer control signal.

FIG. 9 depicts an example embodiment of a ladar transmitter incombination with a dichroic photodetector.

FIG. 10A depicts an example embodiment where the ladar receiver employscorrelation as a match filter to estimate a delay between pulsetransmission and pulse detection.

FIG. 10B depicts a performance model for the example embodiment of FIG.10A.

FIG. 11A depicts an example embodiment of a receiver that employs afeedback circuit to improve the SNR of the sensed light signal.

FIG. 11B depicts another example embodiment relating to the feedbackcircuit design.

FIG. 12 depicts an example process flow for an intelligently-controlledadaptive ladar receiver.

FIG. 13A depicts an example ladar receiver embodiment.

FIG. 13B depicts a plot of signal-to-noise ratio (SNR) versus range fordaytime use of the FIG. 13A ladar receiver embodiment as well asadditional receiver characteristics.

FIG. 14A depicts another example ladar receiver embodiment.

FIG. 14B depicts a plot of SNR versus range for daytime use of the FIG.14A ladar receiver embodiment as well as additional receivercharacteristics.

FIG. 15 depicts an example of motion-enhanced detector arrayexploitation.

FIG. 16 depicts plots showing motion-enhanced detector array trackingperformance.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1A illustrates an example embodiment of a ladartransmitter/receiver system 100. The system 100 includes a ladartransmitter 102 and a ladar receiver 104, each in communication withsystem interface and control 106. The ladar transmitter 102 isconfigured to transmit a plurality of ladar pulses 108 toward aplurality of range points 110 (for ease of illustration, a single suchrange point 108 is shown in FIG. 1A). Ladar receiver 104 receives areflection 112 of this ladar pulse from the range point 110. Ladarreceiver 104 is configured to receive and process the reflected ladarpulse 112 to support a determination of range point distance andintensity information. Example embodiments for innovative ladarreceivers 104 are described below.

In an example embodiment, the ladar transmitter 102 can take the form ofa ladar transmitter that includes scanning mirrors and uses a rangepoint down selection algorithm to support pre-scan compression (whichcan be referred herein to as “compressive sensing”), as shown by FIG.1B. Such an embodiment may also include an environmental sensing system120 that provides environmental scene data to the ladar transmitter tosupport the range point down selection. Example embodiments of suchladar transmitter designs can be found in U.S. patent application Ser.No. 62/038,065, filed Aug. 15, 2014 and U.S. Pat. App. Pubs.2016/0047895, 2016/0047896, 2016/0047897, 2016/0047898, 2016/0047899,2016/0047903, and 2016/0047900, the entire disclosures of each of whichare incorporated herein by reference. For example, incorporated U.S.Pat. App Pub. 2016/0047895 discloses a ladar transmitter that employs adynamic scan pattern where the beam scanner will not scan through a fullscan area, and instead the mirrors will target the range points on ashot list in accordance with a scan pattern that varies as a function ofthe ordered range points on the shot list. Because the shot list will bevarying from frame to frame captured by the environmental sensing system120, the scan pattern is dynamic as it will also vary from frame toframe. As a further example where the ladar transmitter has an X-axismirror and a Y-axis mirror, the driving waveform that can be used todrive the mirror positions for the X-axis mirror can drive the X-axismirror in a resonant mode, and the driving waveform that can be used todrive the Y-axis mirror can drive the Y-axis mirror in a point-to-pointmode where the driving waveform varies as function of the shot list.Through the use of pre-scan compression, such a ladar transmitter canbetter manage bandwidth through intelligent range point targetselection.

FIG. 2 illustrates an example block diagram for an example embodiment ofa ladar receiver 104. The ladar receiver comprises detection optics 200that receive light that includes the reflected ladar pulses 112. Thedetection optics 200 are in optical communication with a light sensor202, and the light sensor 202 generates signals indicative of the sensedreflected ladar pulses 112. Signal read out circuitry 204 reads thesignals generated by the sensor 202 to generate signal data that is usedfor data creation with respect to the range points (e.g., computingrange point distance information, range point intensity information,etc.). It should be understood that the ladar receiver 104 may includeadditional components not shown by FIG. 2. FIGS. 3A-5B show variousexample embodiments of detection optics 200 that may be used with theladar receiver 104. The light sensor 202 may comprise an array ofmultiple individually addressable light sensors (e.g., an n-elementphotodetector array). As an example embodiment, the light sensor 202 cantake the form of a silicon PIN array (e.g., an InGaAs PIN array). Asanother example embodiment, the light sensor 202 can take the form of asilicon avalanche photodiode (APD) array (e.g., an InGaAs APD array).The readout circuitry 204 can take any of a number of forms (e.g., aread out integrated circuit (ROTC)), and example embodiments for thereadout circuitry are described below.

FIG. 3A illustrates an example embodiment of detection optics 200 for aladar receiver 104 which employs a non-imaging light collector 302.Thus, the non-imaging light collector 302 such as a compound parabolicconcentrator, does not re-image the image plane at its entrance fixedpupil 304 onto the light sensor 202 with which it is bonded at its exitaperture. With such an example embodiment, a lens 300 that includes animaging system for focusing light is in optical communication with thenon-imaging light collector 302. In the example of FIG. 3A, the lens 300is positioned and configured such that the lens focuses light (imageplane) at the entrance pupil 304 of the light collector 302 even thoughthere is no actual image at the bonded light sensor.

FIG. 3B illustrates another example embodiment of detection optics 200which employ a non-imaging light collector 302. With such an exampleembodiment, an afocal lens group 310 is in optical communication withthe non-imaging light collector 302. The light collector 302 includes anentrance pupil 304, and it can be bonded with the light sensor 202 atits exit aperture. In the example of FIG. 3B, the lens 310 is positionedand configured such that the entrance pupil of the afocal lens group isre-imaged at the entrance pupil 304 of the light collector 302. Theinventor also notes that if desired by a practitioner, the FIG. 3Bembodiment may omit the afocal lens 310.

With the example embodiments of FIGS. 3A and 3B, the light collector 302can take forms such as a fiber taper light collector or a compoundparabolic concentrator. An example fiber taper light collector isavailable from Schott, and an example compound parabolic concentrator isavailable from Edmunds Optics.

The example embodiments of FIGS. 3A and 3B provide various benefits topractitioners. For example, these example embodiments permit the use ofrelatively small detector arrays for light sensor 202. As anotherexample, these embodiments can be useful as they provide a practitionerwith an opportunity to trade detector acceptance angle for detector sizeas well as trade SNR for high misalignment tolerance. However, theembodiments of FIGS. 3A and 3B do not produce optimal SNRs relative toother embodiments.

FIG. 4 illustrates an example embodiment of detection optics 200 whichemploy an imaging light collector 320. Thus, the imaging light collector320 re-images the image received at its entrance pupil 304 onto thelight sensor 202. With such an example embodiment, a lens 300 thatincludes an imaging system for focusing light is in opticalcommunication with the imaging light collector 320. The lens ispositioned and configured such that the lens focuses light (image plane)at the entrance pupil 304 of the light collector 302, and the lightcollector 320 images this light onto the bonded light sensor 202. In anexample embodiment, the light collector 320 can take the form of acoherent fiber taper light collector. An example coherent fiber taperlight collector is available from Schott.

The example embodiment of FIG. 4 also provides various benefits topractitioners. For example, as with the examples of FIGS. 3A and 3B, theexample embodiment of FIG. 4 permits the use of relatively smalldetector arrays for light sensor 202. This embodiment can also be usefulfor providing a practitioner with an opportunity to trade detectoracceptance angle for detector size as well as trade SNR for highmisalignment tolerance. A benefit of the FIG. 4 example embodimentrelative to the FIGS. 3A/3B example embodiments is that the FIG. 4example embodiment generally produces higher SNR.

FIG. 5A illustrates an example embodiment of “direct to detector”detection optics 200 for a ladar receiver 104. With such an exampleembodiment, a lens 300 that includes an imaging system for focusinglight is in optical communication with the light sensor 202. The lens300 is positioned and configured such that the lens focuses light (imageplane) directly onto the light sensor 202. Thus, unlike the embodimentof FIGS. 3A and 4, there is no light collector between the lens 300 andthe light sensor 202.

FIG. 5B illustrates another example embodiment of “direct to detector”detection optics 200 for a ladar receiver 104. With such an exampleembodiment, an afocal lens 310 is in optical communication with thelight sensor 202. The lens 310 is positioned and configured such thatthe lens pupil is re-imaged directly onto the light sensor 202. Theinventor also notes that if desired by a practitioner, the FIG. 5Bembodiment may omit the afocal lens 310.

The example embodiments of FIGS. 5A and 5B are expected to require alarger detector array for the light sensor 202 (for a given system fieldof view (FOV) relative to other embodiments), but they are also expectedto exhibit very good SNR. As between the embodiments of FIGS. 5A and 5B,the embodiment of FIG. 5A will generally exhibit better SNR than theembodiment of FIG. 5B, but it is expected that the embodiment of FIG. 5Bwill generally be more tolerant to misalignment (which means the FIG. 5Bembodiment would be easier to manufacture).

It should also be understood that the detection optics 200 can bedesigned to provide partial imaging of the image plane with respect tothe light sensor 202 if desired by a practitioner. While this wouldresult in a somewhat “blurry” image, such blurriness may be suitable fora number of applications and/or conditions involving low fill factordetector arrays.

FIG. 6A illustrates an example embodiment for readout circuitry 204within a ladar receiver that employs a multiplexer 604 for selectingwhich sensors 602 within a detector array 600 are passed to a signalprocessing circuit 606. In this example embodiment, the light sensor 202takes the forms of a detector array 600 comprising a plurality ofindividually-addressable light sensors 602. Each light sensor 602 can becharacterized as a pixel of the array 600, and each light sensor 602will generate its own sensor signal 610 in response to incident light.Thus, the array 600 can comprise a photodetector with a detection regionthat comprises a plurality of photodetector pixels. The embodiment ofFIG. 6A employs a multiplexer 604 that permits the readout circuitry 204to isolate the incoming sensor signals 610 that are passed to the signalprocessing circuit 606 at a given time. In doing so, the embodiment ofFIG. 6A provides better received SNR, especially against ambient passivelight, relative to ladar receiver designs such as those disclosed byU.S. Pat. No. 8,081,301 where no capability is disclosed for selectivelyisolating sensor readout. Thus, the signal processing circuit 606 canoperate on a single incoming sensor signal 610 (or some subset ofincoming sensor signals 610) at a time.

The multiplexer 604 can be any multiplexer chip or circuit that providesa switching rate sufficiently high to meet the needs of detecting thereflected ladar pulses. In an example embodiment, the multiplexer 604multiplexes photocurrent signals generated by the sensors 602 of thedetector array 600. However, it should be understood that otherembodiments may be employed where the multiplexer 604 multiplexes aresultant voltage signal generated by the sensors 602 of the detectorarray 600. Moreover, in example embodiments where a ladar receiver thatincludes the readout circuitry 204 of FIG. 6A is paired with a scanningladar transmitter that employs pre-scan compressive sensing (such as theexample embodiments employing range point down selection that aredescribed in the above-referenced and incorporated patent applications),the selective targeting of range points provided by the ladartransmitter pairs well with the selective readout provided by themultiplexer 604 so that the receiver can isolate detector readout topixels of interest in an effort to improve SNR.

A control circuit 608 can be configured to generate a control signal 612that governs which of the incoming sensor signals 610 are passed tosignal processing circuit 606. In an example embodiment where a ladarreceiver that includes the readout circuitry 204 of FIG. 6A is pairedwith a scanning ladar transmitter that employs pre-scan compressivesensing according to a scan pattern, the control signal 612 can causethe multiplexer to selectively connect to individual light sensors 602in a pattern that follows the transmitter's shot list (examples of theshot list that may be employed by such a transmitter are described inthe above-referenced and incorporated patent applications). The controlsignal 612 can select sensors 602 within array 600 in a pattern thatfollows the targeting of range points via the shot list. Thus, if thetransmitter is targeting pixel x,y with a ladar pulse, the multiplexer604 can generate a control signal 612 that causes a readout of pixel x,yfrom the detector array 600. FIG. 8 shows an example embodiment forcontrol circuit 608. The control circuit 608 receives the shot list 800as an input. This shot list is an ordering listing of the pixels withina frame that are to be targeted as range points by the ladartransmitter. At 802, the control circuit selects a first of the rangepoints/target pixels on the shot list. At 804, the control circuit mapsthe selected range point to a sensor/pixel (or a compositepixel/superpixel) of the detector array 600. At 806, the control circuitthen generates a control signal 612 that is effective to cause themultiplexer to readout the mapped sensor/pixel (or compositepixel/superpixel) of the detector array 600. At 808, the control circuitprogresses to the next range point/target pixel on the shot list andreturns to operation 802. If necessary, the control circuit 608 caninclude timing gates to account for round trip time with respect to theladar pulses targeting each pixel.

It should be understood that the control signal 612 can be effective toselect a single sensor 602 at a time or it can be effective to selectmultiple sensors 602 at a time in which case the multiplexer 604 wouldselect a subset of the incoming sensor signals 610 for furtherprocessing by the signal processing circuit 606. Such multiple sensorscan be referred to as composite pixels (or superpixels). For example,the array 600 may be divided into an J×K grid of composite pixels, whereeach composite pixel is comprised of X individual sensors 602. Summercircuits can be positioned between the detector array 600 and themultiplexer 604, where each summer circuit corresponds to a singlecomposite pixel and is configured to sum the readouts (sensor signals610) from the pixels that make up that corresponding composite pixel.

It should also be understood that a practitioner may choose to includesome pre-amplification circuitry between the detector array 600 and themultiplexer 604 if desired.

FIG. 6B depicts an example ladar receiving method corresponding to theexample embodiment of FIG. 6A. At step 620, a ladar pulse is transmittedtoward a targeted range point. As indicated above, a location of thistargeted range point in a scan area of the field of view can be known bythe ladar transmitter. This location can be passed from the ladartransmitter to the ladar receiver or determined by the ladar receiveritself, as explained below.

At step 622, a subset of pixels in the detector array 600 are selectedbased on the location of the targeted range point. As indicated inconnection with FIG. 8, a mapping relationship can be made betweenpixels of the detector array 600 and locations in the scan area suchthat if pixel x1,y1 in the scan area is targeted, this can be translatedto pixel j1,k1 in the detector array 600. It should be understood thatthe subset may include only a single pixel of the detector array 600,but in many cases the subset will comprise a plurality of pixels (e.g.,the specific pixel that the targeted range point maps to plus somenumber of pixels that surround that specific pixel). Such surroundingpixels can be expected to also receive energy from the range point ladarpulse reflection, albeit where this energy is expected to be lower thanthe specific pixel.

At step 624, the selected subset of pixels in the detector array 600senses incident light, which is expected to include thereflection/return of the ladar pulse transmitted at step 620. Each pixelincluded in the selected subset will thus produce a signal as a functionof the incident sensed light (step 626). If multiple pixels are includedin the selected subset, these produced pixel-specific signals can becombined into an aggregated signal that is a function of the incidentsensed light on all of the pixels of the selected subset. It should beunderstood that the detector pixels that are not included in theselected subset can also produce an output signal indicative of thelight sensed by such pixels, but the system will not use these signalsat steps 626-630. Furthermore, it should be understood that the systemcan be configured to “zero out” the pixels in the selected subset priorto read out at steps 624 and 626 eliminate the effects of anystray/pre-existing light that may already be present on such pixels.

At step 628, the photodetector signal generated at step 626 isprocessed. As examples, the photodetector signal can be amplified anddigitized to enable further processing operations geared towardresolving range and intensity information based on the reflected ladarpulse. Examples of such processing operations are discussed furtherbelow.

At step 630, range information for the targeted range point is computedbased on the processing of the photodetector signal at step 628. Thisrange computation can rely on any of a number of techniques. Also, thecomputed range information can be any data indicative of a distancebetween the ladar system 100 and the targeted range point 110. Forexample, the computed range information can be an estimation of the timeof transit for the ladar pulse 108 from the transmitter 102 to thetargeted range point 110 and for the reflected ladar pulse 112 from thetargeted range point 110 back to the receiver 104. Such transit timeinformation is indicative of the distance between the ladar system 100and the targeted range point 110. For example, the range computation canrely on a measurement of a time delay between when the ladar pulse wastransmitted and when the reflected ladar pulse was detected in thesignal processed at step 628. Examples of techniques for supporting suchrange computations are discussed below.

It should be understood that the process flow of FIG. 6B describes anadaptive ladar receiving method where the active sensing region of thedetector array 600 will change based on where the ladar pulses aretargeted by the ladar transmitter. In doing so, it is believed thatsignificant reductions in noise and improvements in range resolutionwill be achieved. Further still, as explained in greater detail below,the subset of detector pixels can be adaptively selected based oninformation derived from the sensed light to further improveperformance.

Returning to FIG. 6A, the signal processing circuit 606 can beconfigured to amplify the selected sensor signal(s) passed by themultiplexer 604 and convert the amplified signal into processed signaldata indicative of range information and/or intensity for the ladarrange points. Example embodiments for the signal processing circuit 606are shown by FIGS. 7A and 7B.

In the example of FIG. 7A, the signal processing circuit 606 comprisesan amplifier 700 that amplifies the selected sensor signal(s), ananalog-to-digital converter (ADC) 702 that converts the amplified signalinto a plurality of digital samples, and a field programmable gate array(FPGA) that is configured to perform a number of processing operationson the digital samples to generate the processed signal data.

The amplifier 700 can take the form of a low noise amplifier such as alow noise RF amplifier or a low noise operational amplifier. The ADC 702can take the form of an N-channel ADC.

The FPGA 704 includes hardware logic that is configured to process thedigital samples and ultimately return information about range and/orintensity with respect to the range points based on the reflected ladarpulses. In an example embodiment, the FPGA 704 can be configured toperform peak detection on the digital samples produced by the ADC 702.In an example embodiment, such peak detection can be effective tocompute range information within +/−10 cm. The FPGA 704 can also beconfigured to perform interpolation on the digital samples where thesamples a curve fit onto a polynomial to support an interpolation thatmore precisely identifies where the detected peaks fit on the curve. Inan example embodiment, such interpolation can be effective to computerange information within +/−5 mm.

When a receiver which employs a signal processing circuit such as thatshown by FIG. 7A is paired with a ladar transmitter that employscompressive sensing as described in the above-referenced andincorporated patent applications, the receiver will have more time toperform signal processing on detected pulses because the ladartransmitter would put fewer ladar pulses in the air per frame than wouldconventional transmitters, which reduces the processing burden placed onthe signal processing circuit. Moreover, to further improve processingperformance, the FPGA 704 can be designed to leverage the parallelhardware logic resources of the FPGA such that different parts of thedetected signal are processed by different hardware logic resources ofthe FPGA at the same time, thereby further reducing the time needed tocompute accurate range and/or intensity information for each rangepoint.

Furthermore, the signal processing circuit of FIG. 7A is capable ofworking with incoming signals that exhibit a low SNR due to the signalprocessing that the FPGA can bring to bear on the signal data in orderto maximize detection.

In the example of FIG. 7B, the signal processing circuit 606 comprisesthe amplifier 700 that amplifies the selected sensor signal(s) and atime-to-digital converter (TDC) 710 that converts the amplified signalinto a plurality of digital samples that represent the sensed light(including reflected ladar pulses). The TDC can use a peak and holdcircuit to detect when a peak in the detected signal arrives and alsouse a ramp circuit as a timer in conjunction with the peak and holdcircuit. The output of the TDC 710 can then be a series of bits thatexpresses timing between peaks which can be used to define rangeinformation for the range points.

The signal processing circuit of FIG. 7B generally requires that theincoming signals exhibit a higher SNR than the embodiment of FIG. 7A,but the signal processing circuit of FIG. 7B is capable of providinghigh resolution on the range (e.g., picosecond resolution), and benefitsfrom being less expensive to implement than the FIG. 7A embodiment.

FIG. 9 discloses an example embodiment where the ladar transmitter 102and a photodetector 900 are used to provide the ladar receiver 104 withtracking information regarding where the ladar transmitter (via itsscanning mirrors) is targeted. In this example, photodetector 900 ispositioned optically downstream from the scanning mirrors (e.g., at theoutput from the ladar transmitter 102), where this photodetector 900operates as (1) an effectively transparent window for incident lightthat exhibits a frequency within a range that encompasses thefrequencies that will be exhibited by the ladar pulses 108 (where thisfrequency range can be referred to as a transparency frequency range),and (2) a photodetector for incident light that exhibits a frequencythat is not within the transparency frequency range. Thus, thedoped/intrinsic layer and the substrate of the photodetector can bechosen so that the ladar pulses 108 fall within the transparencyfrequency range while light at another frequency is absorbed anddetected. The region of the photodetector that exhibits this dualproperty of transmissiveness versus absorption/detection based onincident light frequency can be housed in an opticallytransparent/transmissive casing. The electronic circuitry ofphotodetector 900 that supports the photodetection operations can behoused in another region of the photodetector 900 that need not betransparent/transmissive. Such a photodetector 900 can be referred to asa dichroic photodetector.

The ladar transmitter 102 of FIG. 9 is equipped with a second lightsource (e.g., a second bore-sighted light source) that outputs light 902at a frequency which will be absorbed by the photodetector 900 andconverted into a photodetector output signal 904 (e.g., photocurrent q).Light 902 can be laser light, LED light, or any other light suitable forprecise localized detection by the photodetector 900. The ladartransmitter 102 can align light 902 with ladar pulse 108 so that thescanning mirrors will direct light 902 in the same manner as ladar pulse108. The photodetector's output signal 904 will be indicative of the x,yposition of where light 902 strikes the photodetector 900. Due to thealignment of light 902 with ladar pulse 108, this means that signal 904will also be indicative of where ladar pulse 108 struck (and passedthrough) the photodetector 900. Accordingly, signal 904 serves as atracking signal that tracks where the ladar transmitter is targeted asthe transmitter's mirrors scan. With knowledge of when each ladar pulsewas fired by transmitter 102, tracking signal 904 can thus be used todetermine where the ladar transmitter was aiming when a ladar pulse 108is fired toward a range point 110. We discuss below how timing knowledgeabout this firing can be achieved. Tracking signal 904 can then beprocessed by a control circuit in the ladar receiver 104 or otherintelligence within the system to track where ladar transmitter 102 wastargeted when the ladar pulses 108 were fired. By knowing preciselywhere the transmitter is targeted, the system is able to get improvedposition location of the data that is collected by the receiver. Theinventors anticipate that the system can achieve 1 mrad or better beampointing precision for a beam divergence of around 10 mrad. This allowsfor subsequent processing to obtain position information on the rangepoint return well in excess of the raw optical diffraction limit.

We will now discuss time of transmit and time of receipt for laserlight. FIG. 10A discloses an example embodiment where an optical pathdistinct from the path taken by ladar pulse 108 from the transmitter 102toward a range point and back to the receiver 104 via ladar pulsereflection 112 is provided between the ladar transmitter 102 and ladarreceiver 104, through which reference light 1000 is communicated fromtransmitter 102 to receiver 104, in order to improve range accuracy.Furthermore, this distinct optical path is sufficient to ensure that thephotodetector 600 receives a clean copy of the reference light 1000.

This distinct optical path can be a direct optical path from thetransmitter 102 to the receiver's photodetector 600. With such a directoptical path, the extra costs associated with mirrors or fiber optics toroute the reference light 1000 to the receiver's photodetector 600 canbe avoided. For example, in an arrangement where the transmitter andreceiver are in a side-by-side spatial arrangement, the receiver 104 caninclude a pinhole or the like that passes light from the transmitter 102to the photodetector 600. In practice this direct optical path can bereadily assured because the laser transmit power is considerablystronger than the received laser return signal. For instance, at 1 km,with a 1 cm receive pupil, and 10% reflectivity, the reflected lightsensed by the receiver will be over 1 billion times smaller than thelight at the transmitter output. Hence a small, um scale, pinhole in theladar receiver casing at 104, with the casing positioned downstream fromthe output of mirror 904 would suffice to establish this direct link. Inanother embodiment, a fiber optic feed can be split from the main fiberlaser source and provide the direct optical path used to guide thereference light 1000, undistorted, onto the photodetector.

The reference light 1000, spawned at the exact time and exact locationas the ladar pulse 108 fired into the environment, can be the same pulseas ladar pulse 108 to facilitate time delay measurements for use inrange determinations. In other words, the reference light 1000 comprisesphotons with the same pulse shape as those sent into the field. However,unlike the ladar pulse reflection from the field, the reference lightpulse is clean with no noise and no spreading.

Thus, as shown in the example expanded view of the ladar receiver 104 inFIG. 10A, the photodetector 600 receives the reference pulse 1000 viathe distinct optical path and then later the reflected ladar pulse 112.The signal sensed by the photodetector 600 can then be digitized by anADC 1002 and separated into two channels. In a first channel, a delaycircuit/operator 1004 delays the digitized signal 1006 to produce adelayed signal 1008. The delayed signal 1008 is then compared with thedigitized signal 1006 via a correlation operation 1010. This correlationoperation can be the multiplication of each term 1006, 1008 summedacross a time interval equal to or exceeding the (known) pulse length.As signal 1006 effectively slides across signal 1008 via the correlationoperation 1010, the correlation output 1012 will reach a maximum valuewhen the two signals are aligned with each other. This alignment willindicate the delay between reference pulse 1000 and reflected pulse 112,and this delay can be used for high resolution range determination. Forexample, suppose, the reference light signal 1000 arrives 3 digitalsamples sooner than the reflected ladar pulse 112. Assume these twosignals are identical (no pulse spreading in the reflection), and equal,within a scale factor, {1,2,1}, i.e. the transmit pulse lasts threesamples. Then for a delay of zero in 1004, summing twice the pulselength, the output is {1,2,1,0,0,0} times {0,0,0,1,2,1}. Next suppose wedelay by 1 sample in 1004. Then the output is sum[{0,1,2,1,0,0} times{0,0,0,1,2,1}]=1. If we increment the delay by 1 sample again, we get 4as the correlation output 1012. For the next sample delay increment, weget a correlation output of 6. Then, for the next sample delayincrement, we get a correlation output of 4. For the next two sampledelay increments we get correlation outputs of 1 and then zerorespectively. The third sample delay produces the largest correlationoutput, correctly finding the delay between the reference light and thereflected ladar pulse. Furthermore, given that for a range of 1 km, thetransmitter can be expected to be capable of firing 150,000 pulses everysecond, it is expected that there will be sufficient timing space forensuring that the receiver gets a clean copy of the reference light 1000with no light coming back from the ladar pulse reflection 112. The delayand correlation circuit shown by FIG. 10A can also be referred to as amatched filter. The matched filter can be implemented in an FPGA orother processor that forms part of signal processing circuit 606.

While the example of FIG. 10A shows a single photodetector 600 and ADC1002 in the receiver, it should be understood that separatephotodetectors can be used to detect the return pulse 112 and thereference pulse 1000. Also, separate ADCs could be used to digitize theoutputs from these photodetectors. However, it is believed that the useof a single photodetector and ADC shared by the return pulse 112 andreference pulse 114 will yield to cost savings in implementation withoutloss of performance. Also, interpolation of the sampled return pulse 112can be performed as well using pulse 1000 as a reference. After peakfinding, conducted using the process described above, the system canfirst interpolate the reference light signal. This can be done using anydesired interpolation scheme, such as cubic spline, sine functioninterpolation, zero pad and FFT, etc. The system then interpolates thereceive signal around the peak value and repeats the process describedabove. The new peak is now the interpolated value. Returning to ourprevious example, suppose we interpolate the reference light pulse toget {1,1.5,2,1.5,1,0,0,0,0,0,0}, and we interpolate the receive pulselikewise to get {0,0,0,1,1,5,2,1,5,1}. Then the system slides,multiplies, and sums. The advantage of this, over simply “trusting” theladar return interpolation alone, is that the correlation with thereference light removes noise from the ladar return.

Making reference pulse 1000 the same as ladar pulse 108 in terms ofshape contributes to the improved accuracy in range detection becausethis arrangement is able to account for the variation in pulse 108 fromshot to shot. Specifically, range is improved from the shape, andreflectivity measurement is improved by intensity, using pulse energycalibration (which is a technique that simply measures energy ontransmit). The range case is revealed in modeling results shown by FIG.10B. The vertical axis of FIG. 10B is range accuracy, measured as ±x cm,i.e. x standard deviations measured in cm, and the horizontal axis ofFIG. 10B is the SNR. This model is applied to a 1 ns full width halfmaximum Gaussian pulse. The bottom line plotted in FIG. 10B is the idealcase. The nearby solid line 121 is the plot for an ADC with 1 picosecondof timing jitter, which is a jitter level readily available commerciallyfor 2 GHz ADCs. By comparing the performance of the two curves indicatedbelow 121, one can see from FIG. 10B that jitter is not a limitingfactor in achieving sub-cm resolution. Specifically the lower curve (nojitter) and upper curve (jitter) differ by only a millimeter at veryhigh (and usually unachievable) SNR [˜1000]. However, pulse variation isa significant limitation. This is seen by 120, which is the performanceavailable with 5% pulse-to-pulse shape variation, a common limit incommercial nanosecond-pulsed ladar systems. The difference between 120and 121 is the improvement achieved by example embodiments of thedisclosed FIG. 10A technique, for both peak finding and interpolation asa function of SNR.

We conclude the discussion of range precision by noting that thecomputational complexity of this procedure is well within the scope ofexisting FPGA devices. In one embodiment, the correlation andinterpolation can be implemented after a prior threshold is crossed bythe data arriving from the reflected lidar pulse. This greatly reducescomplexity, at no performance cost. Recall, the intent of correlationand interpolation is to improve ranging—not detection itself, sodelaying these operations and applying them only around neighborhoods ofdetected range returns streamlines computations without erodingperformance. Typically only 3 samples are taken of the reference lightpulse since it is so short. Interpolating this 20-fold using cubicmodels requires only about 200 operations, and is done once per shot,with nominally 100,000 shots. The total burden pre matching filter andinterpolation against the ladar receive pulse is then 20 Mflops. If weselect the largest, first and last pulse for processing, this rises toless than 100 Mflop, compared to teraflops available in moderncommercial devices.

Furthermore, FIG. 11A discloses an example embodiment of a receiverdesign that employs a feedback circuit 1100 to improve the SNR of thesignals sensed by the active sensors/pixels 602. The feedback circuit1100 can be configured as a matching network, in resonance with thereceived ladar pulse return 112 (where the ladar pulse 108 and returnpulse 112 can exhibit a Gaussian pulse shape in an example embodiment),thereby enhancing the signal and retarding the noise. A photodetectorperformance is a function of pitch (area of each element) and bandwidth.Passive imagers lack prior knowledge of incident temporal signalstructure and have thus no ability to tune performance. However, inexample embodiments where the ladar transmitter employs compressivesensing, the transmitted ladar pulse 108 is known, as it is arrival timewithin the designated range swath. This knowledge can facilitate amatching network feedback loop that filters the detector current,increases signal strength, and filters receiver noise. A feedback gainprovided by the feedback circuit can be controlled via a control signal1102 from control circuit. Furthermore, it should be understood that thecontrol circuit 608 can also be in communication with the signalprocessing circuit 606 in order to gain more information about operatingstatus for the receiver.

The matching; network of the feedback circuit 1100 may be embedded intothe In—GaAs substrate of detector 600 to minimize RF coupling noise andcross channel impedance noise. The cost of adding matching networks ontothe detector chip is minimal. Further, this matching allows us to obtainbetter dark current, ambient light, and Johnson noise suppression thanis ordinarily available. This further reduces required laser power,which, when combined with a 1.5 um wavelength for ladar pulses 108leads, to a very eye safe solution. The matching network can becomprised of more complex matching networks with multiple poles,amplifiers, and stages. However, a single pole already providessignificant benefits. Note that the input to the signal processingcircuit 606 can be Gaussian, regardless of the complexity of themultiplexer, the feedback, or the size variability of the pixels, due tothe convolutional and multiplicative invariance of this kernel.

FIG. 11B shows an example that expands on how the feedback circuit 1100can be designed. The matching network involves one or more amplifiers1102, in a controlled feedback loop 1104 with a gain controllerfurnished by the control circuit 608. The matching network can bepresent on all the input lines to the mux 604, and FIG. 11B shows justshow a single such network, within the dotted box 1120, for ease ofillustration. The feedback gain is generally chosen to output maximalSNR using differential equations to model the input/output relationshipsof the feedback circuit. In practice the control loop can be designed tomonitor the mux output and adjust the amplifiers 1102 to account fromdrift due to age, thermal effects, and possible fluctuations in ambientlight. Embodiments are also disclosed herein which employ two or moredigital channels to build a filter (e.g. a Weiner filter or least meansquares filter) to reject interference from strong scatterers, otherladar pulses, or even in-band sunlight, headlights or othercontaminants. Also, the feedback circuit can be reset at each shot toavoid any saturation from contamination in the output from shot to shot.

Feedback control can be vastly simplified if a Gaussian pulse shape isused for ladar pulse 108 in which case all the space time signals remainnormally distributed, using the notation in 1122. Accordingly, in anexample embodiment, the ladar pulse 108 and its return pulse 112 canexhibit a Gaussian pulse shape. In such an example embodiment (where thelaser pulse 108 is Gaussian), the Fourier representation of the pulse isalso Gaussian, and the gain selection by, the control circuit 608 istractable, ensuring rapid and precise adaptation.

Another innovative aspect of the design shown by FIG. 11B is the use ofhexagonally shaped pixels for a plurality of the sensors 602 within thephotodetector array 600. The shaded area 1130 indicates the selectedsubset of pixels chosen to pass to the signal processing circuit 606 ata given time. By adaptively selecting which pixels 602 are selected bythe multiplexer 604, the receiver can grow or shrink the size of theshaded area 1130, either by adding or subtracting pixels/sensors 602.The hexagonal shape of pixels/sensors 602 provides a favorable shape forfault tolerance since each hexagon has 6 neighbors. Furthermore, thepixels/sensors 602 of the photodetector array 600 can exhibit differentsizes and/or shapes if desired by a practitioner. For example, some ofthe pixels/sensors can be smaller in size (see 1132 for example) whileother pixels/sensors can be larger in size (see 1134). Furthermore, somepixels/sensors can be hexagonal, while other pixels/sensors can exhibitdifferent shapes.

FIG. 12 depicts an example process flow for implementing adaptivecontrol techniques for controlling how the receiver adapts the activeregion of the photodetector array 600. At step 1200, a list of pixelseligible for inclusion in subset 1130 is defined. This list can be anydata structure 1202 that includes data indicative of which pixels 602are eligible to be selected for inclusion in the subset 1130. Such adata structure may be maintained in memory that is accessible to aprocessor that implements the FIG. 12 process flow. While the example ofFIG. 12 shows a list 1202 that identifies eligible pixels 602, it shouldbe understood that data structure 1202 could also serve as an effectiveblacklist that identifies pixels that are ineligible for inclusion insubset 1130.

At step 1204, a circuit (e.g., signal processing circuit 606 and/orcontrol circuit 608), which may include a processing logic (e.g., anFPGA) and/or other processor, operates to derive information from thelight sensed by the array 600 (which may be sensed by a subset of pixels602 that are active in the array) or from the environmental scene (e.g.,by processing camera/video images). This derived information may includeinformation such as whether any saturation conditions exist, whether anypixels are malfunctioning, whether there are any areas of high noise inthe field of view, etc. Examples of derived information that can beuseful for adaptive control are discussed below. Furthermore, it shouldbe understood that the oversaturation conditions can be attributed tospecific pixels (e.g., pixels that are blinded by intense incidentlight) and/or can be attributed to the aggregated signal resulting fromthe combination of pixel readings by the pixels included in subset 1130(where the aggregation of pixel outputs oversaturates the linearoperating range of the processing circuitry).

At step 1206, the list of eligible pixels 1202 is adjusted based on theinformation derived at step 1204. For example, if a given pixel is foundto be malfunctioning as a result of step 1204, this pixel can be removedfrom list 1202 at step 1206. Similarly, any oversaturated pixels can beremoved from the list 1202 and/or any pixels corresponding to overlynoisy areas of the field of view (e.g., regions where the noise exceedsa threshold) can be removed from list 1202 at step 1206.

Next, at step 1208, the system selects pixels from the list 1202 ofeligible pixels based on the targeted range point. This can be performedas described in connection with step 804 of FIG. 8, but where list 1202defines the pool of pixels eligible to be selected as a function of thelocation of the targeted range point in the scan area/field of view.Thus, if the targeted range point is mapped to pixel 1140 in the arrayand the subset 1130 would have ordinarily included all of the pixelsthat neighbor pixel 1140, the adaptive control technique of FIG. 12 mayoperate to define subset 1130 such that the upper left neighboring pixelof pixel 1140 is not included in subset 1130 if the upper leftneighboring pixel was removed from list 1202 at step 1206 (e.g., due toa detected malfunction or the like). Furthermore, it should beunderstood that step 1208 may also operate to use the informationderived at step 1204 to affect which eligible pixels are included in thesubset. For example, additional pixels might be added to the subset 1130to increase the size of the active sensor region based on the derivedinformation. Similarly, the size of the active sensor region might beshrunk by using fewer pixels in the subset 1130 based on the derivedinformation. Thus, it should also be understood that the size of theactive region defined by the selected subset 1130 may fluctuate fromshot to shot based on information derived at step 1204.

At step 1210, the pixels selected at step 1208 are included in subset1130, and the MUX is then controlled to read/combine the outputs fromthe pixels that are included in the selected subset 1130 (step 1212).Thereafter, the process flow returns to step 1204 for the next ladarpulse shot. Accordingly, it can be seen that the process flow of FIG. 12defines a technique for intelligently and adaptively controlling whichpixels in array 600 are used for sensing ladar pulse returns.

Furthermore, it should be understood that the FIG. 12 process flow canalso be used to impact transmitter operation. For example, the list ofeligible pixels (or a list of ineligible pixels) can be provided to theladar transmitter for use by the ladar transmitter to adjust thetiming/order of shots on its shot lists (e.g., avoiding shots that willlikely be corrupted by noise on receive). Further still, as an example,if the information derived at step 1204 indicates that the aggregatedsignal produced by MUX 604 is oversaturated, the ladar transmitter canreduce the power used by the ladar pulses 108 to reduce the likelihoodof oversaturation on the receive side. Thus, when such oversaturationcorrupts the receiver, the ladar transmitter can repeat the corruptedshot by reducing the power for ladar pulse 108 and re-transmitting thereduced power pulse.

Also disclosed herein are specific examples of control techniques thatcan be employed by the ladar system. While each control technique willbe discussed individually and should be understood as being capable ofimplementation on its own, it should also be understood that multiplesof these control techniques can be aggregated together to furtherimprove performance for the adaptive receiver. As such, it should beunderstood that in many instances aggregated combinations of thesecontrol techniques will be synergistic and reinforcing. In other cases,tradeoffs may exist that are to be resolved by a practitioner based ondesired operating characteristics for the receiver.

Adaptive Fault Tolerance Mask:

With a conventional imaging array, a dead pixel typically leads toirrecoverable loss. However, with the adaptive control featuresdescribed herein, a malfunctioning pixel 602 has minimal effect. Supposefor example that we have an array 600 of 500 pixels 602. Then suppose wehave a lens that maps the far field scene to a 7-pixel super/compositepixel 1130 (a specified pixel 1140 and its neighbors). Losing one pixelleads to a loss of 1/7 of the net photon energy. If the detector arrayis shot noise-limited, then we have only a 7% loss in energy, versus100% loss for a full imaging array. An example control flow for a faulttolerant adaptive mask is shown below as applied to an embodiment wherethe ladar transmitter employs compressive sensing. It should beunderstood that a mask can be used by the control circuit 608 to definewhich pixels 602 are included in the selected subset of active sensorsand which are not so included. For example, the mask can be a datasignal where each bit position corresponds to a different pixel in thearray 600. For bit positions having a value of “1”, the correspondingpixel 602 will be included in the selected subset, while for bitpositions having a value of “0”, the corresponding pixel 602 would notbe included in the selected subset.

A pixel 602 that is unable to detect light (i.e., a “dead” pixel or a“dark” pixel) should not be included in the selected subset because sucha dead pixel would add noise but no signal to the aggregated sensedsignal corresponding to the composite pixel defined by the selectedsubset. Furthermore, it should be understood that malfunctioning pixelsare not limited to only dead pixels. A pixel 602 that produces an outputsignal regardless of whether incident light is received (e.g., a “stuck”pixel or a “white” pixel) should also be omitted from the selectedsubset. In fact, a white pixel may be even worse than a dark pixelbecause the stuck charge produced by the white pixel can lead to aconstant bright reading which adds glare to all returns in the compositepixel. An example control process flow is described below for generatingan adaptive fault tolerant mask that can adjust which pixels 602 areincluded in the selected subset based on which pixels 602 are detectedas malfunctioning:

-   -   1: select a background pixel status probe shot schedule        repetition rate (e.g., nominally one per hour)    -   2: Decompose: In the past previous time block T identify S, the        set of pixels not yet selected for illumination. Decompose into        S1, S2, the former being addressable (strong return in scene)        while the latter is defined to be non-addressable (ex: above        horizon). Note that S1, S2 are time varying.    -   3: Shot list: Enter S1, S2 into the shot list.    -   4: construct a mask to deselect faulty tiles identified from        analysis of returns from 1-3 (either no return or anomalous        gain), The super-pixel size can be set based on the lens and        tile pitch but can nominally be 7.    -   5: recurse 1-4.    -   6: average: In the above, as necessary, apply running averages        on pixel probe, and include adaptive metrology.

Fault tolerance in this fashion can be a useful step in improvingsafety, since without mitigation single defects can render an entire FOVinoperative.

Adaptive Mask to Control Dynamic Range:

The adaptive control over which subsets of pixels are activated at agiven time can also be used to adjust the dynamic range of the system.Based on range knowledge, the signal produced by a composite pixel willhave predictable intensity. A mask can be constructed that reduces (orincreases) the dynamic range of the return at the ADC pre-filter and/orthe ADC itself by adjusting the size of the composite pixel defined bythe pixels 602 included in the selected subset. For example, if thetypical composite pixel is 7 pixels (see 1130 in FIG. 11B), adjustingthe subset such that it drops from 7 pixels to a single pixel reducesthe energy by 7 times (or roughly three bits). Photodetectors measureenergy of light, not amplitude of light. As a result, the ADCs dynamicrange is the square of that for conventional communications and radarcircuits which measure amplitude. As a result, properly controllingdynamic range is a technical challenge for laser systems. For example, aladar system tuned to operate over 10-500 m will, for a fixedreflectivity and laser power, undergo a signal return dynamic rangechange by 2500. If a nearby object saturates the receiver, a farther outtarget will be lost. Therefore, an example embodiment can include ananalysis of prior shot range returns in the instantaneous field of viewto assess the need to excise any pixels from the selected subset in themux circuit. As a result, there may be a desire for having the MUX dropthe sensor signal(s) from one or more pixels of the region 1130, asoutlined below. An example control process flow is described below forgenerating an adaptive mask for controlling the dynamic range of thereturn signal:

-   -   1, Inspect range return from a pulse return of interest,        obtained from either selective or compressive sensing.    -   2. Identify any saturation artifacts, as evidenced by ADC        reports at the MSB (most significant bit) for several range        samples.    -   3. Map the saturated range sample to a precise azimuth and        elevation of origin. This may involve exploring adjacent cells        to determine origin from context, particularly at longer range        as beam divergence is more pronounced.    -   4. Modify the mask to reduce saturation by blocking the pixels        that present a larger gain in the origin identified in 3.    -   5. Modify the mask further by selecting only smaller area pixels        as required.

Adaptive Mask to Remove Interfering Ladar Pulse Collisions:

Another potential source of noise in the light sensed by the receiver isa collision from an interfering ladar pulse. For example, in anapplication where the ladar system is employed on moving automobiles,the incoming light that is incident on the photodetector array 600 mightinclude not only a ladar pulse return 112 from the vehicle that carriesthe subject ladar system but also a ladar pulse or ladar pulse returnfrom a different ladar system carried by a different vehicle (aninterfering “off-car” pulse). Adaptive isolation of such interferingpulses can be achieved by creating a sub-mask of selected pixels 602 byexcising pixels associated with strong interfering pulses from otherladar systems. The above-referenced and incorporated patent applicationsdescribe how pulse encoding can be employed to facilitate the resolutionas to which ladar pulses are “own” pulses and which are “off” pulses(e.g., “off-car” pulses). For example, consider that such encoding isused to detect that pixel 1134 contains energy from an interfering ladarpulse. We would then scan through the pixels of the array (with thecluster 1130 for example) to see which are receiving interference. Inone embodiment, this would involve removing the “own” lidar pulse usingencoding, measuring the resulting signal after subtraction, andcomparing to a predetermined threshold. In another embodiment, thesystem would simply analyze the MUX output, subtract off the “own” pulseencoding signal and compare the remainder to a threshold. The embodimentwill depend on the severity of interference encountered, and processorresources that are available. Upon such detection, the control circuit608 can remove this pixel 1134 from a list of eligible pixels forinclusion in a selected subset while the interfering pulse is registeredby that pixel 1132.

The system might also remove pixels based on headlight sourcelocalization from passive video dining night time operations (theoperational conservative assumption here being that every vehicle with aheadlight has a ladar transmitter). Furthermore, since pulse collisiondetection can be used to reveal off-car pulses, this information can beused to treat any selected off car laser source as a desired signal,subtract off the rest (including own-car ladar pulses) and scan throughpixels of the array to find where this interference is largest. In doingso we will have identified the source of each interfering ladar source,which can then be subsequently removed.

Adaptive Mask for Strong Scatterer Removal:

Another potential source of noise in the light sensed by the receiver iswhen a ladar pulse strikes an object that exhibits a strong scatteringeffect (e.g., a strongly slanted and reflective object as opposed to amore ideally-oriented object that is perpendicular to the angle ofimpact by the ladar pulse 108). Targets exhibiting multiple returns haveinformation bearing content. However, this content can be lost due toexcessive dynamic range, because the largest return saturates drivingthe receiver into nonlinear modes, and/or driving the weaker returnsbelow the sensor detection floor. Typically, the direct return is thelargest, while successive returns are weakened by the ground bouncedispersion, but this is not the case when reflectivity is higher inbounce returns. In either case, it is desirable to adjust the mask sothat the near-in range samples receive a higher pupil (dilation) (e.g.,where the selected subset defines a larger area of the array 600), whilethe farther out range samples undergo pupil contraction (e.g., where theselected subset defines a smaller area of the array 600). At far rangethere will be large angular extent for the laser spot. It is possiblefor strong near-range scatterer pulse returns to arrive within the dataacquisition window for the transmitted pulse. The use of an adaptivemask will allow for the removal of this scatterer by over-resolving thespot beam (e.g., more than one pixel covered by the shot return beam) onreceive, thereby reducing saturation or scatterer leakage into thetarget cell. For example suppose, notionally we observe that the rangereturns begin at 1134, migrate to the doublet at 1132 and at closestrange appear at 1130. We can then instruct the control circuit to modifythe mask by choosing different mux lines as the laser pulse sweepsacross the sensor array.

Adaptive Shot Timing Linked to Mask Feedback Control:

In compressive sensing, the dynamic range can be further reduced bydeliberately timing the laser pulse by the transmitter so that the laserpeak intensity does not fall on the target but instead falls away fromnear-to-the-target interference, thereby increasing the signal toclutter ratio. This allows for near-in interference suppression aboveand beyond that obtained by other means. For example, suppose,notionally, that the upper sensor cell 1132 contains a very strongtarget and the lower nearby sensor cell also labeled 1132 contains atarget. Then we can set the shot timing to move the received pulse shotillumination away from the 1132 doublet and center it more towards 1130.We are using here the flexibility in shot timing (provided viacompressive sensing), knowledge of beam pointing on transmit (see FIG.9), and selectivity in sensor elements (see FIG. 11B, for example) tooptimally tune the receiver and transmitter to obtain the best possiblesignal quality. By ensuring the mask is tuned so that the beam peak ofthe receive beam is away from a noise source (e.g., incoming traffic) wecan reduce strong returns from nearby vehicles while imaging atdistance, a milliradian in some cases suffices to reduce strongscatterers by 95% while attenuating the target object by only a fewpercent. In an example embodiment, selective sensing can be used todetermine the mask parameters, although compressive sensing, or fixedroadmap-based solutions may also be chosen. An example here is lanestructure, since opposing lane traffic yields the largest interferencevolume. The system could thereby adjust the shots, or the ordering ofshots to avoid noisy areas while retaining the desired objectinformation.

Adaptive Mask for Dynamic Range Mitigation by Mask Mismatch:

If the mask in 1130 is chosen to provide the largest ladar reflectionmeasurement, the center pixel will have the most energy. Therefore itwill saturate before any of the others. Therefore one approach forreducing saturation risk is to simply remove the center pixel from themask 1130 if evidence of, or concern regarding, saturation is present.

Adaptive Mask for Power-Coherent interference Rejection:

One benefit of the advanced receiver disclosed herein is that only asingle data channel is needed, as opposed to M where M is the pixelcount. However, one can still retain a low cost and swap system byadding a second channel. This second channel, like the first channel,can either be a full up analog to digital converter (see FIG. 7A) or atime of flight digitizer (see FIG. 7B). Either embodiment allows forcoherent combining (in intensity) to optimally suppress the interferenceusing filtering (such as Weiner Filtering or Least Means Squared (LMS)Filtering). With two channels x,y and with the target return weightingbeing w_(x), w_(y), this is equivalent to solving for the weights andapplying the weights to the data so that the SNR of w_(x)x+w_(y)y ismaximized. Through such an adaptive mask, the spatially directionalnoise component in the sensed light signal can be reduced.

The embodiments of FIGS. 6A-12 can be particularly useful when pairedwith detection optics such as those shown by FIGS. 4 and 5A, where thesensed light is imaged onto the detector array 600. In embodiments wherethe image pulse is not imaged onto the detector array 600 (e.g., theembodiments of FIGS. 3A, 3B, and 5B (or embodiments where the image is“blurry” due to partial imaging), then a practitioner may choose to omitthe multiplexer 604 as there is less of a need to isolate the detectedsignal to specific pixels.

FIG. 13A depicts an example ladar receiver embodiment where “direct todetector” detection optics such as that shown by FIG. 5A are employedand where the readout circuitry of FIG. 7A is employed. In this example,the ladar receiver is designed with an approximately 60×60 degree FOV,and an approximate 150 m range (@SNR=8, 10% reflectivity). The receiveremploys a low number N-element detector array such as a silicon orInGaAs PIN/APD array. When using an InGaAs PIN array, the receiver mayexhibit a 2 cm input aperture, a 14 mm focal length, and it may work inconjunction with an approximately 0.2-5.0 nanosecond laser pulse ofaround 4 microJoules per pulse. Spatial/angular isolation may be used tosuppress interference, and a field lens may be used to ensure that thereare no “dead spots” in the detector plane in case the detectors do nothave a sufficiently high fill factor. FIG. 13B depicts a plot of SNRversus range for daytime use of the FIG. 13A ladar receiver embodiment.FIG. 13B also shows additional receiver characteristics for thisembodiment. Of note, the range at reflectivity of 80% (metal) is over600 m. Furthermore, the max range envelope is between around 150 m andaround 600 m depending on real life target reflectivities andtopography/shape.

FIG. 14A depicts an example ladar receiver embodiment where detectionoptics such as that shown by FIG. 3B are employed and where the readoutcircuitry of FIG. 7A is employed. In this example, the ladar receiver isdesigned with an approximately 50×50 degree FOV, and an approximate 40 mrange (@SNR=8, 10% reflectivity). As with the embodiment of FIG. 13A,the receiver employs a low number N-element detector array such as asilicon or InGaAs PIN/APD array. When using an InGaAs PIN array, thereceiver of FIG. 14A may exhibit a 2 cm input aperture, employ an afocalnon-imaging lens, and it may work in conjunction with an approximately0.2-5.0 nanosecond laser pulse of around 4 microJoules per pulse. FIG.14B depicts a plot of SNR versus range for daytime use of the FIG. 14Aladar receiver embodiment. FIG. 14B also shows additional receivercharacteristics for this embodiment. Of note, the range at reflectivityof 80% (metal) is around 180 m. Furthermore, the max range envelope isbetween around 40 m and around 180 m depending on real life targetreflectivities and topography/shape.

It is also possible to dramatically improve the detection range, the SNRand therefore detection probability, or both, by exploiting motion ofeither a ladar system-equipped vehicle or the motion of the objects itis tracking, or both. This can be especially useful for mapping a roadsurface due to a road surface's low reflectivity (˜20%) and the pulsespreading and associated SNR loss.

The stochastic modulation of the two way (known) beam pattern embedsposition information on the point cloud(s) obtained. We can extract fromthis embedding improved parameter estimates. This is essentially thedual of ISAR (inverse synthetic aperture radar) in radar remote sensing.This is shown in FIG. 15, where we show the detector output for a givenazimuth and elevation pixel, with each row being the range returns froma single shot. As we aggregate shots we obtain integration gain. In FIG.15 the solid white curve 1502 shows how a specified, fixed, groundreference point varies vertically due to vehicle motion. Note that themotion can lead to a non-linear contour. This is due to the fact that,even for fixed velocity, the ground plane projection does not, at nearrange, present a planar projection. In other words, the Jacobian of theground plane projection is parametrically variant. The relative motionexploitation that we propose is to integrate the detector array outputs,either binary or intensity, along these contours to recreate the groundplane map. Such integration is necessitated in practice by the fact thatthe pulse spreads and thus each shot will present weak returns. Further:the asphalt tends to have rather low reflectivity, on the order of 20%,further complicating range information extraction. The white rectangularregion 1502 show the migration in shots for a vehicle presentingrelative motion with respect to the laser source vehicle. To simplifythe plot, we show the case where differential inter-velocity [closingspeed] is constant. The width of the rectangle 1502 presents theuncertainty in this differential. The scale shows this width is muchlarger than the width for ground mapping described above. This isbecause we must estimate the differential speed using ladar, while theown-car has GPS, accelerometers, and other instrumentation to enhancemetrology. Close inspection will show that inside the white tiltedrectangle 1502 there are more detections. This example is for an SNR of2, showing that, even at low SNR, an integration along track [binary]can provide adequate performance. The receiver operating curve can bereadily computed and is shown in FIG. 16. Shown is the detectionprobability, 1600 (thin lines upper right) as well as the false alarmcurve, bottom left, 1602. We move for thin lines from one shot to 30shots. The horizontal axis is the threshold at the post integrationlevel, forming lines in the kinematic space as per FIG. 15. At athreshold of 1.5 observe that we get 95% 6% Pd Pfa at 15 shots, whichfor a closing speed of 50 m/s is 25 m target vehicle ingress, or ½second.

While the invention has been described above in relation to its exampleembodiments, various modifications may be made thereto that still fallwithin the invention's scope. Such modifications to the invention willbe recognizable upon review of the teachings herein.

What is claimed is:
 1. A ladar system comprising: a ladar transmitterthat includes a beam scanner, wherein the beam scanner selectivelytargets a plurality of range points within a frame, and wherein theladar transmitter transmits ladar pulses toward the selectively targetedrange points in accordance with a shot list; and a ladar receiver thatincludes a photodetector array and a circuit; wherein the photodetectorarray comprises a plurality of light sensors, wherein each light sensor(1) senses light that is indicative of a plurality of ladar pulsereturns from a plurality of the range points and (2) generates a signalindicative of the sensed light; wherein the circuit selectively definesa plurality of subsets of the light sensors for read out at a given timebased on the shot list to produce a signal representative of the sensedlight signals from the light sensors included within the definedsubsets, the produced signal for use in computing range information withrespect to the targeted range points; and wherein the circuit includesfeedback circuitry that amplifies outputs from the light sensors in acontrolled feedback loop.
 2. The system of claim 1 wherein the beamscanner selectively targets the range points by dynamically scanning amirror according to the shot list.
 3. The system of claim 2 wherein thebeam scanner comprises a first mirror and a second mirror, wherein thefirst mirror is scannable to a plurality of first mirror scan positions,wherein the second mirror is scannable to a plurality of second mirrorscan positions, wherein the first and second mirror scan positions incombination define where the ladar transmitter is targeted, and whereinthe beam scanner employs compressive sensing to selectively target asubset of range points in the frame based on a range point downselection algorithm that identifies a subset of range points in theframe for targeting with ladar pulses.
 4. The system of claim 2 whereinthe beam scanner comprises a first mirror and a second mirror, whereinthe first mirror is scannable to a plurality of first mirror scanpositions based on a first control signal, wherein the second mirror isscannable to a plurality of second mirror scan positions based on asecond control signal, wherein the first control signal scans the firstmirror in a resonant mode, wherein the second control signal scans thesecond mirror in a point-to-point mode that varies as a function of theshot list, and wherein the first and second mirror scan positions incombination define where the ladar transmitter is targeted.
 5. Thesystem of claim 1 wherein the circuit includes a multiplexer thatcontrols which light sensors are included in the defined subsets basedon a control signal, wherein the control signal varies based on the shotlist.
 6. The system of claim 5 wherein the feedback circuitry includesamplifiers that are operatively between the photodetector array and themultiplexer.
 7. The system of claim 6 wherein the photodetector array isresident on a substrate, and wherein the amplifiers are embedded in thesubstrate.
 8. The system of claim 5 wherein the feedback circuitryserves as a matching network in resonance with a received ladar pulsereturn.
 9. The system of claim 8 wherein the matching network is presenton all input lines to the multiplexer.
 10. The system of claim 1 whereinthe light sensors correspond to pixels, and wherein the circuitselectively defines the subsets based on a mapping relationship betweenthe pixels and range point locations that are targeted for ladar pulseshots by the shot list.
 11. The system of claim 1 wherein the feedbackcircuitry serves as a matching network in resonance with a receivedladar pulse return.
 12. The system of claim 1 wherein the feedbackcircuitry is reset at each ladar pulse shot.
 13. The system of claim 1wherein the light sensors of the photodetector array correspond to aplurality of pixels, and wherein the defined subsets of light sensorschange over time with respect to how many pixels are included in thedefined subsets.
 14. The system of claim 1 wherein the light sensors ofthe photodetector array correspond to a plurality of pixels, and whereinthe circuit selectively controls which pixels are eligible for inclusionin the defined subsets based on feedback with respect to prior frames.15. The system of claim 1 wherein the ladar transmitter transmits aplurality ladar pulses that exhibit a Gaussian pulse shape.
 16. A ladarsystem comprising: a ladar transmitter that includes a beam scanner,wherein the beam scanner selectively targets a plurality of range pointswithin a frame, and wherein the ladar transmitter transmits ladar pulsestoward the selectively targeted range points in accordance with a shotlist; and a ladar receiver that includes a photodetector array, amultiplexer, and a feedback circuit; wherein the photodetector arraycomprises a plurality of light sensors, wherein each light sensor (1)senses light that is indicative of a plurality of ladar pulse returnsfrom a plurality of the range points and (2) generates a signalindicative of the sensed light; wherein the multiplexer selectivelydefines a plurality of subsets of the light sensors for read out at agiven time a control signal that varies based on the shot list toproduce a signal representative of the sensed light signals from thelight sensors included within the defined subsets, the produced signalfor use in computing range information with respect to the targetedrange points; wherein the feedback circuit amplifies outputs from thelight sensors in a controlled feedback loop; and wherein the multiplexerreceives the amplified outputs from the light sensors.
 17. A ladarsystem comprising: a ladar transmitter that selectively targets aplurality of range points within a frame in accordance with a shot list;and a ladar receiver that includes a photodetector array and a circuit;wherein the photodetector array comprises a plurality of light sensors,wherein each light sensor (1) corresponds to a pixel, (2) senses lightthat is indicative of a plurality of ladar pulse returns from aplurality of the range points, and (3) generates a signal indicative ofthe sensed light; wherein the circuit selectively defines a plurality ofsubsets of the light sensors for read out at a given time based on theshot list to produce a signal representative of the sensed light signalsfrom the light sensors included within the defined subsets, the producedsignal for use in computing range information with respect to thetargeted range points; and wherein the circuit selectively controlswhich pixels are eligible for inclusion in the defined subsets based onfeedback with respect to prior frames.
 18. The system of claim 17wherein the ladar transmitter includes a beam scanner that selectivelytargets the range points by dynamically scanning a mirror according tothe shot list.
 19. The system of claim 18 wherein the beam scannercomprises a first mirror and a second mirror, wherein the first mirroris scannable to a plurality of first mirror scan positions, wherein thesecond mirror is scannable to a plurality of second mirror scanpositions, wherein the first and second mirror scan positions incombination define where the ladar transmitter is targeted, and whereinthe beam scanner employs compressive sensing to selectively target asubset of range points in the frame based on a range point downselection algorithm that identifies a subset of range points in theframe for targeting with ladar pulses.
 20. The system of claim 18wherein the beam scanner comprises a first mirror and a second mirror,wherein the first mirror is scannable to a plurality of first mirrorscan positions based on a first control signal, wherein the secondmirror is scannable to a plurality of second mirror scan positions basedon a second control signal, wherein the first control signal scans thefirst mirror in a resonant mode, wherein the second control signal scansthe second mirror in a point-to-point mode that varies as a function ofthe shot list, and wherein the first and second mirror scan positions incombination define where the ladar transmitter is targeted.
 21. Thesystem of claim 17 wherein the circuit generates an adaptive faulttolerance mask that adjusts which of the pixels are eligible forinclusion in the defined subsets based on feedback from prior framesthat indicates whether any of the pixels are malfunctioning.
 22. Thesystem of claim 17 wherein the circuit generates an adaptive mask thatcontrols dynamic range for the produced signal by adjusting how manypixels are to be included in the defined subsets based on feedback fromprior frames.
 23. The system of claim 17 wherein the circuit generatesan adaptive mask that adjusts which of the pixels are eligible forinclusion in the defined subsets based on feedback from prior framesthat indicates a presence of interfering light that would impact one ormore of the pixels.
 24. The system of claim 17 wherein the circuitgenerates an adaptive mask that adjusts which of the pixels are eligiblefor inclusion in the defined subsets based on feedback from prior framesthat indicates a presence of a scattering object that would impact oneor more of the pixels.
 25. The system of claim 17 wherein the circuitadjusts timing for ladar pulse shots on the shot list based on feedbackfrom prior frames to reduce interference on the pixels of the definedsub sets.
 26. The system of claim 17 wherein the circuit adjusts shotenergy for ladar pulse shots on the shot list based on feedback fromprior frames.
 27. The system of claim 17 wherein the circuit adds orremoves ladar pulse shots to or from the shot list based on feedbackfrom prior frames.
 28. The system of claim 17 wherein the circuitgenerates an adaptive mask that removes a center pixel from the definedsubsets to reduce saturation risk based on feedback from prior frames.29. The system of claim 17 wherein the feedback from prior framescomprises data derived from prior ladar pulse returns.
 30. The system ofclaim 17 wherein the prior frames comprise video frames.